Add archive note to the README.md

[jtpio]

I am currently not able to give this project proper attention and maintenance effort, and would then like to archive the repo.

cc @felixchenier @Drixs @garmann @OliPlus @bmihelac since you have open PRs on the repo.

I'll leave this PR opened for a couple of weeks to let folks comment here if they would like to take over.

If someone wants to maintain a fork or a copy, feel free! I'll update the PR to point to the new repo. If you want to continue publishing to PyPI, then I would suggest choosing a different name for the Python package.

Thanks!

[bmihelac]

@jtpio - thanks for your time and effort you invested in this project.